UNIVERSITA’ DEGLI STUDI DI PARMA Dipartimento di

Crittografia applicataCrittografia applicata::

X.509 e PGPX.509 e PGP

LucaLuca VeltriVeltri

((mailmail..toto:: [email protected]@uniprunipr..itit))

Corso di Sicurezza nelle reti di telecomunicazioni, a.a. 2011/2012Corso di Sicurezza nelle reti di telecomunicazioni, a.a. 2011/2012

httphttp://://wwwwww..tlctlc..uniprunipr..itit/veltri/veltri

UNIVERSITA’ DEGLI STUDI DI PARMA

Dipartimento di Ingegneria dell’Informazione

CertificatiCertificati e Certification Authoritye Certification Authority

X.509 & PGPX.509 & PGPUniversità degli Studi di Parma

Dipartimento di Ingegneria dell'Informazione

Università degli Studi di ParmaUniversità degli Studi di Parma

Dipartimento di Ingegneria dell'InformazioneDipartimento di Ingegneria dell'Informazione X.509 & PGPX.509 & PGP

3

Key DistributionKey Distribution

� Public key cryptography solves a major problem with symmetric

algorithms

� I can encrypt messages to you with your public key

� You can verify my signatures using my public key

� Requires no pre-established relationship between us

� But how do you get my public key?

� And how do you know it is my public key?

� PGP and X.509/PKI

� PGP uses a web of of trust

� X.509/PKI uses hierarchical CAs





4

Public Key Infrastructure (PKI)Public Key Infrastructure (PKI)

� System for publishing and verifying the public key values used in

public key cryptography

� Certification binds a public-key value to an individual, organizationor other entity

• Performed by Certification Authorities (CAs)

� Validation is the process of verifying that a certification is stillvalid

• Performed by end users or systems





5

Certificato digitaleCertificato digitale

� I certificati sono documenti digitali che attestano la

corrispondenza di una chiave pubblica con un

individuo/organizzazione

� Nella forma più semplice contengono

� una chiave pubblica e il nome dell’individuo/organizzazione,

� il numero di serie del certificato e le date di emissione/scadenza

� il nome di chi ha emesso il certificato (Certification Authority)

� la firma digitale di chi ha emesso il certificato (viene utilizzata lachiave privata del soggetto/organizzazione che emette ilcertificato)

� Il formato maggiormente diffuso e accettato è basato sullo

standard ITU-T X.509





6

Where Certificates Are DeployedWhere Certificates Are Deployed

� Web transactions� Transport Layer Security (TLS)

• Old version called Secure Sockets Layer (SSL)

� Virtual Private Networks� IPSEC using Internet Key Exchange (IKE)

� Secure messaging� S/MIME, Pretty Good Privacy (PGP)

� Anywhere strong authentication and/or encryption is required





7

Certificate AuthorityCertificate Authority

InternetInternetEntity A Entity B

C E R T I F I C A T E A U T H O R I T Y

� La Certificate Authority (CA) svolge la funzione di certificare le

chiavi pubbliche

� garantisce “la connessione” tra le chiavi e l’entità a cui siriferiscono

� Questa operazione di certificazione avviene attraverso

l’emissione di un certificato digitale





8

Certificate Authority (Certificate Authority (contcont.).)

� L’entità certificata può essere:

� una persona

� una organizzazione

� un ruolo all’interno di una organizzazione

� uno pseudonimo

� un sistema hardware

� codice software

� Alcune Autorità garantiscono la certificazione solo ad unsottoinsieme di entità

� Ad una Autorità di certificazione possono essere riposti livelli di

fiducia diversi





9

CertificationCertification AuthorityAuthority ((contcont.).)CA1 (root)

CA2

CA4

CA3

CA5 CA6 CA7





10

Types of CertificatesTypes of Certificates

� CA signed certificate

� la firma digitale del certificato viene apposta da una CertificationAuthority

� Self signed certificate

� la firma digitale del certificato viene apposta dal proprietario dellachiave pubblica





11

Types of CertificatesTypes of Certificates

� Root Certificates

� Self-signed by a Certification Authority

� CA Certificates

� For verifying signatures on issued certificates

� End systems certificates

� e.g.

• Server Certificates

– For use by SSL/TLS servers

• Software Signing Certificates

– For signing executable code

X.509X.509





13

X.509 PKIX.509 PKI

� PKIX Working Group dell’IETF definisce una PKI come

� L’insieme di hardware, software, persone e procedure che sononecessarie per la creazione, la gestione, la memorizzazione, ladistribuzione e la revoca di certificati digitali impiegati in unsistema di crittografia a chiave pubblico-privata

� IETF, RFC 2459: “Internet X.509 Public Key Infrastructure

Certificate and CRL Profile”

� descrive il formato dei certificati X.509 v3, le estensioni di uncertificato, il formato delle CRL, altro

� IETF, RFC 2527: “Internet X.509 Public Key Infrastructure

Certificate Policy and Certification Practices Framework”

� Fornisce indicazioni (framework) per la stesura del CertificationPractice Statements (CPS) a cura di una Autorità di certificazione





14

X.509 HistoryX.509 History

� ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was

first published in 1988 as part of the X.500 Directory recommendations,

defines a standard certificate format

� The certificate format in the 1988 standard is called the version 1 (v1)format

� When X.500 was revised in 1993, two more fields were added, resulting inthe version 2 (v2) format

� The Internet Privacy Enhanced Mail (PEM) RFCs, published in 1993, includespecifications for a PKI based on X.509 v1 certificates

� The experience gained in attempts to deploy PEM RFCs made it clear thatthe v1 and v2 certificate formats are deficient in several respects

� ISO/IEC/ITU and ANSI X9 developed the X.509 version 3 (v3)

certificate format (June 1996)

� The v3 format extends the v2 format by adding provision for additionalextension fields





15

X.509X.509

� While trust can be assigned to individual keys, the power of the

X.509 model comes from its default arrangement of delegating

the trust decision to the certification authority

� It does this by assuming trust is inherited from the signing key

� Vendors of X.509 products generally include a set of root

certificates that the product will trust “out of the box”

� therefore automatically validate other certificates presented to theproduct

� X.509 encryption and signature capabilities are built into manyweb browsers and mail programs

� for example the secure HTTP protocol (HTTPS) used for web-based ordering and on-line banking uses X.509





16

Certificato digitale X.509Certificato digitale X.509

� Contiene:

� una chiave pubblica

� il riferimento (Distinguished Name) ad informazioni che associanola chiave pubblica ad il suo proprietario (nome, e-mail, il nomedella Società, telefono) od al dispositivo (Security gateway) che nefa uso (Indirizzo IP, …)

� il riferimento (Distinguished Name) ad informazioni circa l’organoche ha emesso il certificato (nome, e-mail, telefono)

� un serial number che identifica univocamente il certificato

� un indicatore del livello di trust garantito

� la data di emissione del certificato

� la data di scadenza del certificato

� la firma digitale dell’organo che ha emesso il certificato





17

Certificate signatureCertificate signature

Message

Digest

Hashing

Algorithm

Issuer’s

Private

Key

Subject’s Public Key

SubjectC=US ST=NY L=Albany O=OFT CN=John Doe

Period of Validity

IssuerC=US O=RSA Security OU=Secure Certification

Authority

Signature Algorithm Identifier

Version/Serial Number

Signature of Issuer





18

X.509 v1,v2,v3 and CRL FormatsX.509 v1,v2,v3 and CRL Formats





19

Example of PEM-encoded certificateExample of PEM-encoded certificate

-----BEGIN CERTIFICATE-----

MIIC7jCCAlegAwIBAgIBATANBgkqhkiG9w0BAQQFADCBqTELMAkGA1UEBhMCWFkx

FTATBgNVBAgTDFNuYWtlIERlc2VydDETMBEGA1UEBxMKU25ha2UgVG93bjEXMBUG

A1UEChMOU25ha2UgT2lsLCBMdGQxHjAcBgNVBAsTFUNlcnRpZmljYXRlIEF1dGhv

cml0eTEVMBMGA1UEAxMMU25ha2UgT2lsIENBMR4wHAYJKoZIhvcNAQkBFg9jYUBz

bmFrZW9pbC5kb20wHhcNOTgxMDIxMDg1ODM2WhcNOTkxMDIxMDg1ODM2WjCBpzEL

MAkGA1UEBhMCWFkxFTATBgNVBAgTDFNuYWtlIERlc2VydDETMBEGA1UEBxMKU25h

a2UgVG93bjEXMBUGA1UEChMOU25ha2UgT2lsLCBMdGQxFzAVBgNVBAsTDldlYnNl

cnZlciBUZWFtMRkwFwYDVQQDExB3d3cuc25ha2VvaWwuZG9tMR8wHQYJKoZIhvcN

AQkBFhB3d3dAc25ha2VvaWwuZG9tMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKB

gQDH9Ge/s2zcH+da+rPTx/DPRp3xGjHZ4GG6pCmvADIEtBtKBFAcZ64n+Dy7Np8b

vKR+yy5DGQiijsH1D/j8HlGE+q4TZ8OFk7BNBFazHxFbYI4OKMiCxdKzdif1yfaa

lWoANFlAzlSdbxeGVHoT0K+gT5w3UxwZKv2DLbCTzLZyPwIDAQABoyYwJDAPBgNV

HRMECDAGAQH/AgEAMBEGCWCGSAGG+EIBAQQEAwIAQDANBgkqhkiG9w0BAQQFAAOB

gQAZUIHAL4D09oE6Lv2k56Gp38OBDuILvwLg1v1KL8mQR+KFjghCrtpqaztZqcDt

2q2QoyulCgSzHbEGmi0EsdkPfg6mp0penssIFePYNI+/8u9HT4LuKMJX15hxBam7

dUHzICxBVC1lnHyYGjDuAMhe396lYAn8bCld1/L4NMGBCQ==

-----END CERTIFICATE-----





20

Certification Paths and TrustCertification Paths and Trust

� A user of a security service requiring knowledge of a public key

generally needs to obtain and validate the certificate containing

the required public key

� If the user does not already hold an assured copy of the public

key of the CA that signed the certificate, the CA's name, and

related information (such as the validity period or name

constraints), then it might need an additional certificate to obtain

that public key

� In general, a chain of multiple certificates may be needed,

comprising a certificate of the public key owner (the end entity)

signed by one CA, and zero or more additional certificates of

CAs signed by other CAs





21

Certificate RevocationCertificate Revocation

� When a certificate is issued, it is expected to be in use for its

entire validity period (certificates have a period of validity)

� However, various circumstances may cause a certificate to

become invalid (revoked) prior to the expiration of the validity

period, e.g.

� change of name, change of association between subject and CA(e.g., an employee terminates employment with an organization)

� user's private key is assumed (or suspected) to be compromised

� user is no longer certified by this CA

� CA's certificate is assumed to be compromised

� X.509 defines one method of certificate revocation

� This method involves each CA periodically issuing a signed data

structure called a certificate revocation list (CRL)

� users should check certs with CA’s CRL





22

Certificate Revocation List (CRL)Certificate Revocation List (CRL)

� A CRL is a time stamped list identifying revoked certificates which is

signed by a CA and made freely available in a public repository

� When a system uses a certificate, that system not only checks the

certificate signature and validity but also acquires a suitably-recent CRL

and checks that the certificate serial number is not on that CRL

� A CA issues a new CRL on a regular periodic basis (e.g., hourly, daily,

or weekly)

� An entry is added to the CRL as part of the next update followingnotification of revocation

� An entry may be removed from the CRL after appearing on one regularlyscheduled CRL issued beyond the revoked certificate's validity period

� CRLs may be distributed by exactly the same means as certificates

themselves, namely, via untrusted communications and server

systems

� One limitation of the CRL revocation method, is that the time granularity

of revocation is limited to the CRL issue period





23

How areHow are CAsCAs secure?secure?

� Because they automatically validate other keys, certification

authority signing keys are far more valuable than simple e-

commerce keys or personal keys

� Compromising one of these trusted certification authority keys

would permit an attacker

� to fabricate e-commerce certificates, signature certificates and soforth certificates that would be indistinguishable from legitimatecertificates and that would be automatically trusted by yourbrowser, e-mail application, or other X.509-enabled application

� Certification authority key signing keys have long lifetimes (ten to

30 years) and so are generally 2048 bits to protect them during

that lifetime





24


� It is distressing to see a key such as the following in the trusted

certification authority key store provided by the vendor:





25


� The work to factor a 1024 bit RSA key like this is about the same

as to brute force a 74 bit key

� The cost to do so would be about 86 million dollars, while the

machine to do so in ten years could be built for about 30 million

dollars

� So here we have a hundreds of millions of dollars of value

protected by a lock that can be broken in a timely fashion (that is,

while the key is valid) with a 30 million dollar tool. This is not the

way it is supposed to work

� Note:

� Internet Explorer 5.5 is distributed with 106 trusted rootcertificates, of which over half have only 1024 bit keys — and atleast one of which has only a 1000 bit key





26

Memorizzazione delle chiaviMemorizzazione delle chiavi

� Chiave privata: viene mantenuta memorizzata in maniera crittata

(algoritmo simmetrico) su hard disk, su smartcard o PMCIA card.

L’accesso è protetto da password o passphrase utente

� Chiave pubblica: viene mantenuta all’interno del certificato. I

certificati possono essere memorizzati all’interno di un dispositivo fisico

(Security Gateway) o all’interno di un repository accessibile





27

Problems with X.509 PKI: “Which directory?”Problems with X.509 PKI: “Which directory?”

� The biggest problem is reflected in the simple phrase “fetches a

certificate from a repository” Since the concept of a global

distributed directory (or even a less ambitious local directory)

was never realized, there ’s no clear idea where to fetch a

certificate from, and if you have a certificate there ’s no clear idea

where to fetch its CRL from

� The solution which was adopted, and which works reasonably

well in practice, was to include any certificates which might be

needed wherever they might be needed

� for example, an S/MIME signature usually includes with it all thecertificates needed to verify it, and an SSL server ’scommunication to the client usually includes with it the certificatesneeded to protect those communications





28

Problems with X.509 PKI: “Which John Smith?”Problems with X.509 PKI: “Which John Smith?”

� Even if the user knows which directory to look in, there ’s no way

to determine which DN should be used to find a certificate, or

which of a number of identical names you ’re searching on

belongs to the person whose key you ’re interested in

� PGP solved (?) the problem in a simple manner: users were

allowed to choose any kind of identifier they wanted for

certificates, which generally consisted of an email address





29

Problems with X.509 PKI: CRLProblems with X.509 PKI: CRL

� An entity which doesn’t have a current CRL is expected to fetch the

current one and use that to check the validity of certificates

� In practice this rarely occurs because users and/or applications don’t

know where to go for a CRL, or it takes so long to fetch

� In order to guarantee timely status updates, it’s necessary to issueCRLs as frequently as possible, however the more often a CRL is

issued the higher the load on the server which holds the CRL, on the

network over which it is transmitted, and on the client which fetches it

� creating and distributing them requires processing time, one or more

servers, and significant amounts of network bandwidth (CRLs can

become quite large, and many clients can fetch these large CRLs)

� This problem is addressed by protocols such as OCSP (Online

Certificate Status Protocol) or the SCVP (Simple Certificate ValidationProtocol)





30

Lo standard PKCSLo standard PKCS

� Public-key Cryptography Standard (PKCS)

� It is based on RSA public-key cryptography

� Developed by RSADSI (RSA Data Security Inc.)

� PKCS is actually a set of standards

� describes the syntax for messages in an abstract manner

� gives complete details about algorithms

� defines encoding for

• RSA public/private key,

• signature,

• short RSA-encrypted message (typically a secret key),

• etc





31

PKCS GoalsPKCS Goals

� To maintain compatibility with PEM (the Internet Privacy-

Enhanced Mail protocols, described in RFCs 1421–1424)

wherever possible

� To extend beyond PEM in being able to:

� handle arbitrary binary data (not just ASCII data),

� handle a richer set of attributes and features in (extended)certificates and enveloped data

� To define a standard suitable for incorporation in future OSI

standards

� The standards are based on the use of OSI standard ASN.1(Abstract Syntax Notation One) and BER (Basic Encoding Rules)to describe and represent data





32

Lo standard PKCSLo standard PKCS

PKCS: Public-Key Cryptography Standards

� PKCS #1:RSA Cryptography Standard

� PKCS #3:Diffie-Hellman Key Agreement Standard

� PKCS #5:Password-Based Cryptography Standard

� PKCS #6:Extended-Certificate Syntax Standard

� PKCS #7:Cryptographic Message Syntax Standard

� PKCS #8:Private-Key Information Syntax Standard

� PKCS #9:Selected Attribute Types

� PKCS #10:Certification Request Syntax Standard

� PKCS #11:Cryptographic Token Interface Standard

� PKCS #12:Personal Information Exchange Syntax Standard





33

ASN.1, BER and DERASN.1, BER and DER

� ASN.1 (Abstract Syntax Notation One, defined in X.208) is the OSI's

method of specifying abstract objects

� ASN.1 is a flexible notation that allows one to define a variety data

types

� from simple types such as integers and bit strings to structured types suchas sets and sequences, as well as complex types defined in terms ofothers

� One set of rules for representing such objects as strings of ones andzeros is called the BER (Basic Encoding Rules, defined in X.209)

� BER describes how to represent or encode values of each ASN.1 typeas a string of eight-bit octets

� There is generally more than one way to BER-encode a given value

� Another set of rules, called the Distinguished Encoding Rules (DER),

which is a subset of BER, gives a unique encoding to each ASN.1 value

Pretty GoodPretty Good Privacy (PGP)Privacy (PGP)





35

Pretty Good Privacy (PGP) - HistoryPretty Good Privacy (PGP) - History

� It started out as a single public domain implementation

� Author of PGP is Philip Zimmerman of Guerrilla Freeware

� It was the author's intention that it be distributed widely

� Selected best available crypto algs to use, integrated into a singleprogram

� Both RSADSI (RSA Data Security Inc.) and government authorities

caused PGP to start its life as contraband

� by enforcing respectively the RSA patent, and export control of dangeroustechnologies like nuclear weapons and the ability to encrypt mail

� PGP has been legal and freely available in many other countriesbecause the RSA patent is U.S. only

� other governments have different policies about the export, import, anduse of privacy protection technology

� Originally free, now have commercial versions available also

� different platforms (Windows, Unix/Linux, Macintoch, etc.)





36

GNU PrivacyGNU Privacy GuardGuard ((GnuPGGnuPG))

� Free implementation of the OpenPGP standard as defined by

RFC4880

� RFC4880 describes format and methods needed to read, check,generate, and write conforming packets crossing any network

� Command line tool with features for easy integration with other

applications

� freely available also frontend applications and libraries





37

A PGP MessageA PGP Message

-----BEGIN PGP MESSAGE-----

Version: 2.6.2

hEwDFoFLYIYi6t0BAf0TFjuS2Sp9obCam/Wf1BGsF970sqD0uU9RHNIl4xNny+Sp

U4cwVwDy0HD+CZvdUH+9QAavZwoidlt+AnFqx4uDpgAAAfIxpJ6R06Tou+q6dDTV

ycUXCJKi+DWH/Sfyl+BVo036IFsngt2tE9eoKhZtGzOO0JrC3m1MY/zJFR2FT2kL

uBvGeHSkjC4RlVkoXcbEvIKSyauOBBv4ve1Ze69LVONgBxif8TurmuQpyr0/1+zD

JPClE9M7o5pckQJnuikg8GFc+mkTwiLgIqau518dPUAjwBZQO4wUI0Kj0DKXMgRQ

wd1CMD5EWLjBXvCsNhTCRC/lzVHX1szUjAjIV8m/hMBDVc+VYvBP8KzjBntwFAIf

ql4HnwqdSRebCDODnEKxScrjtIUX9MpTgjoZ49Tg73R1aUl0p4+ALZDu10X6OdgI

JQBMJEsLl8CIo0eKW1NwKTNJBb3nJWpE7LF3X4k5F/PVobwKMsuLZJTOWy/iMCpW

KZJ63B/A0gbHW1ysJpbbNXDyNkHjyQ/N0TkJTxa4FWnrEAzbjfqyDQLH8KDbEA+g

19Wl7y31dziJYjYsGwPZBGzV0vOLh1lkBNw3wHhf2hQLQ6kL5rK4bASMrISha4oe

aFUD30AdcGzl4RgU2U7eElCwaZPucHSq0EM2RCFZs3Rt4fRwcRlEPySEbI9eK9MJ

MNvXRruxPb7wWA1/UHzhJi6GQkT+G4W1EMeo9euwZsfSYsBGiqcz8IIMbG7PfeYH

T+SXD6A=

=f/zW

-----END PGP MESSAGE-----





38

PGP OverviewPGP Overview

� PGP performs authentication, encryption, compression

� Someone wishing to send a secure mail message could first

transform the file to be mailed using PGP, and then mail the

transformed file using a traditional e-mail program

� Similarly, if one were to receive a PGP encrypted mail message,

one could treat the received message as a file and feed it to PGP

to process





39

PGP OverviewPGP Overview





40

The PGP ProcessThe PGP Process

� When you use PGP to encrypt a mail message and send it to a

friend, the following occurs:

� 1. PGP creates a random session key for the message

� 2. PGP uses the IDEA algorithm to encrypt the message with thesession key

� 3. PGP uses the RSA algorithm to encrypt the session key with therecipient's public key

� 4. PGP bundles the encrypted message and the session keytogether and prepares the message for mailing

� PGP handles session keys automatically, without any

intervention on your part





41

PGP OperationsPGP Operations

Consist of five services:

� Authentication

� Confidentiality

� Compression

� Text compatibility

� Segmentation





42

PGP: algoritmi utilizzatiPGP: algoritmi utilizzati

� Generatore di numeri casuali

� Algoritmo simmetrico IDEA, 3DES, etc.

� Algoritmo asimmetrico RSA

� Algoritmo per la compressione dati ZIP

� Algoritmo per la selezione del messaggio MD5 o SHA-1

� Algoritmo per la conversione in formato RADIX-64





43





44

PGP Operation – AuthenticationPGP Operation – Authentication

� sender creates a message

� SHA-1 used to generate 160-bit hash code of message

� hash code is encrypted with RSA using the sender's private key, andresult is attached to message

� receiver uses RSA with sender's public key to decrypt and recoverhash code

� receiver generates new hash code for message and compares withdecrypted hash code, if match, message is accepted as authentic





45

PGP Operation – ConfidentialityPGP Operation – Confidentiality

� sender generates message and random 128-bit (or 168 or 192)number to be used as session key for this message only

� message is encrypted, using CAST-128 / IDEA/3DES with session key

� session key is encrypted using RSA with recipient's public key, thenattached to message

� receiver uses RSA with its private key to decrypt and recover sessionkey

� session key is used to decrypt message





46

PGP Operation – Confidentiality &PGP Operation – Confidentiality &

AuthenticationAuthentication� uses both services on same message

� create signature & attach to message

� encrypt both message & signature

� attach RSA encrypted session key





47

PGP Operation – CompressionPGP Operation – Compression

� by default PGP compresses message after signing but before

encrypting

� so can store uncompressed message & signature for laterverification

� & because compression is non deterministic

� uses ZIP compression algorithm





48

PGP Operation – Email CompatibilityPGP Operation – Email Compatibility

� when using PGP will have binary data to send (encryptedmessage etc)

� however email was designed only for text

� hence PGP must encode raw binary data into printable ASCIIcharacters

� the scheme used is radix-64 conversion� maps 3 bytes to 4 printable chars

� radix-64 expands the message by 33%

� PGP also segments messages if too big

� Often restricted to a maximum message length of 50,000 octets

� Longer messages must be broken up into segments

� PGP automatically subdivides a message that is to large

� The receiver strip of all e-mail headers and reassemble the block





49

Key CertificatesKey Certificates

� PGP keeps each public key in a key certificate. Each key

certificate contains:

� The public key itself

� One or more user IDs for the key's creator (usually that person'sname and e-mail address)

� The date that the key was created

� Optionally, a list of digital signatures on the key, provided bypeople who attest to the key's accuracy





50

PGP Digital SignaturesPGP Digital Signatures





51

PGP Key ManagementPGP Key Management

� rather than relying on certificate authorities

� in PGP every user is own CA� can sign keys for users they know directly

� forms a “web of trust”� trust keys have signed

� can trust keys others have signed if have a chain of signatures tothem

� key ring includes trust indicators

� users can also revoke their keys





52

PGPPGP vsvs. X.509 PKI. X.509 PKI

� X.509 certificates and PGP certificates (often called PGP keys,

for historic reasons) differ in several syntactic ways:

� They use different data formats to encode the elements of thecertificate

� An X.509 certificate contains exactly one public key, whereas aPGP certificate commonly contains at least two public keys—onefor signing and one for encrypting

� An X.509 certificate contains exactly one certification, usually nota self-signed certification

� On the other hand, a PGP certificate contains a collection ofcertifications, usually at least one self-certification and one third-party certification

� While PGP and X.509 are syntactically different, they are

semantically the same.

� This means that appropriately designed software systems can

use either type of certificate in the same processes.

Documents

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